Use for a new modern sporting rifle build, or convert any existing AR-15 to .450 Bushmaster with this complete upper assembly from Great Lakes Firearms and Ammunition. It features an 18" heavy match grade barrel made from chrome moly alloy steel. The A3 style upper is made from aircraft grade aluminum and has an oversized ejection port. The upper comes complete with a free float handguard, low profile gas block, mid length gas system and phosphate coated bolt carrier group.